Acid dianhydride, method for preparing same, and polyimide prepared therefrom

ABSTRACT

The present invention relates to a novel acid dianhydride, a method for preparing the same, and a polyimide prepared therefrom. More specifically, the acid dianhydride according to the present invention is useful as a colorless transparent polyimide unit exhibiting excellent thermal stability and a low dielectric ratio, and the polyimide of the present invention has excellent solubility to an organic solvent compared with the conventional polyimide.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No.PCT/KR2014/004395, filed on May 16, 2014, which claims priority fromKorean Patent Application No. 10-2013-0055391, filed on May 16, 2013,the contents of all of which are incorporated herein by reference intheir entirety.

TECHNICAL FIELD

The present invention relates to an acid dianhydride, a preparationmethod thereof, and a polyimide prepared therefrom.

BACKGROUND ART

Polyimide finds a wide spectrum of applications in the electronicmaterial industry including protecting materials, insulation materials,color filters, etc. for liquid crystal displays and semiconductors byvirtue of its high mechanical strength, thermal resistance, and solventresistance. In addition, expectation has recently been made of the useof polyimide as a material for optical telecommunication and as asubstrate for mobile phones.

With the recent advances in these industries, materials having advancedproperties are increasingly demanded. For example, not only aremechanical properties such as thermal resistance and solvent resistanceneeded, but also functions according to uses, such as transparency,etc., are requested in polyimide for use in this field.

Appearing with a dark amber color, wholly aromatic polyimides forgeneral purposes, which can be obtained by the polycondensation ofaromatic tetracarboxylic dianhydride with aromatic diamine, cannot beapplied where high transparency is needed. In addition, since whollyaromatic polyimide is insoluble in organic solvents, its precursorpolyamic acid is, in practice, employed before film formation throughthermal ring-closing dehydration.

One strategy for achieving transparency is known, wherein aliphatictetracarboxylic dianhydride is polycondensed with aromatic diaminepolyimide to give a polyimide precursor, followed by conversion intopolyimide that is relatively colorless, and highly transparent (JapanesePatent Unexamined Application Publication Nos. Hei 2-24294 and Sho58-208322).

However, the polyamic acids and polyimides that are based onunsubstituted aliphatic tetracarboxylic dianhydride are almost insolublein general organic solvents, and soluble only in polar organic solventswith high boiling points. In this context, a high temperature isemployed upon the formation of the film so as to remove the solvent,exerting undesirable effects on other organic materials of organic ELdevices.

In recent years, a polyimide prepared from the monomer1,2,3,4-cyclopentane tetracarboxylic dianhydride (hereinafter referredto as “CPDA”) has been suggested for use as a gas barrier film oforganic electroluminescence (hereinafter referred to as “organic EL”)(Japanese Patent Unexamined Application Publication No. 2006-232960).

However, this polyimide needs to be improved in thermal resistance andis not of sufficient solubility in organic solvents in addition tohaving a low degree of polymerization.

DISCLOSURE Technical Problem

It is a primary object of the present invention to provide a novel aciddianhydride as a material monomer for a polyimide that has a lowdielectric constant and exhibits excellent thermal stability, solubilityin organic solvents, and light transmittance while retaining theproperties of polyimide itself, and a method for preparing the same.

It is another object of the present invention to provide a polyamic acidcontaining the novel acid dianhydride, and a polyimide prepared throughthe ring-closing dehydration of the polyamic acid.

Technical Solution

According to one aspect for accomplishing the above object, the presentinvention provides an acid dianhydride represented by the followingChemical Formula 1:

In accordance with another aspect thereof, the present inventionprovides a method for preparing an acid dianhydride, represented by thefollowing Chemical Formula 1, comprising: (a) reacting a compoundrepresented by the following Chemical Formula 1 with piperazine to givea compound represented by the following Chemical Formula 3; (b)hydrolyzing the compound of Chemical Formula 3 in the presence of a basecatalyst to obtain a compound represented by Chemical Formula 4; and (c)subjecting the compound of Chemical Formula 4 into ring-closingdehydration in the presence of a dehydrating agent.

In one preferred embodiment of the present invention, the reaction ofstep (a) is a Michael addition reaction.

In another preferred embodiment of the present invention, the compoundof Chemical Formula 2 is reacted at a molar ratio 1:0.45 to 1:0.55 withpiperazine in step (a).

In another preferred embodiment of the present invention, the basecatalyst of step (b) is selected from the group consisting of potassiumhydroxide, sodium hydroxide, barium hydroxide, calcium hydroxide,aluminum hydroxide, magnesium hydroxide, and a combination thereof.

In another preferred embodiment of the present invention, the hydrolysisof step (b) is conducted at 40˜120° C. for 1˜6 hrs.

In another preferred embodiment of the present invention, thedehydrating agent of step (c) is selected from the group consisting ofacetic anhydride, pyridine, isoquinoline, a tertiary amine, and acombination thereof.

In another preferred embodiment of the present invention, thering-closing dehydration of step (c) is conducted at 40˜100° C. for 4˜28hrs.

In another preferred embodiment of the present invention, thedehydrating agent of step (c) is used in an amount of 2˜10 moles permole of the compound of Chemical Formula 4.

A further aspect of the present invention provides a polyamic acid,prepared by reacting an acid dianhydride represented by the followingChemical Formula 1 with diamine:

A still further aspect of the present invention provides a polyimide,prepared from the polyamic acid of claim 10 by ring-closing dehydration.

Advantageous Effects

Provided according to the present invention are a novel acid dianhydrideuseful for the preparation of a colorless transparent polyimide thatexhibits excellent thermal stability and has a low dielectric constantwhile retaining excellent properties of polyimide itself, and a methodfor preparing the same.

Exhibiting excellent thermal stability and having a low dielectricconstant, the colorless transparent polyimide of the present inventionfinds applications in a variety of material industries includingprotecting and insulating materials in liquid crystal display devices orsemiconductors, and materials for optical communication such as opticalwaveguides

MODE FOR INVENTION

Unless otherwise defined, all terms used herein have the same meaning ascommonly understood by one of ordinary skill in the art to which exampleembodiments belong. It will be further understood that terms, e.g.,those defined in commonly used dictionaries, should be interpreted ashaving a meaning that is consistent with their meaning in the context ofthe relevant art and will not be interpreted in an idealized or overlyformal sense unless expressly so defined herein.

It will be further understood that the terms “comprises,” “comprising,”“includes,” and/or “including,” when used herein, specify the presenceof stated features, integers, steps, operations, elements, and/orcomponents, but do not preclude the presence or addition of one or moreother features, integers, steps, operations, elements, components,and/or groups thereof.

An aspect of the present invention addresses an acid dianhydriderepresented by the following Chemical Formula 1:

In accordance with another aspect thereof, the present inventionaddresses a method for preparing an acid dianhydride, represented by thefollowing Chemical Formula 1, comprising: (a) reacting a compoundrepresented by the following Chemical Formula 1 with piperazine to givea compound represented by the following Chemical Formula 3; (b)hydrolyzing the compound of Chemical Formula 3 in the presence of a basecatalyst to obtain a compound represented by Chemical Formula 4; and (c)subjecting the compound of Chemical Formula 4 into ring-closingdehydration in the presence of a dehydrating agent.

As a rule, aliphatic polyimide exhibits higher transparency and is lowerin dielectric constant, compared to aromatic polyimide, because of itslow intramolecular density, polarity, and low inter- or intramolecularcharge transfer. Thanks to these properties, aliphatic polyimide hasattracted intensive attention for use as interlayer insulating materialin optoelectronics.

For use in preparing an aliphatic polyimide that is of high transparencyand low dielectric constant, a nitrogen-containing piperazine-disuccinicanhydride (an acid dianhydride represented by Chemical Formula 1) issynthesized in the present invention.

Since the acid dianhydride represented by Chemical Formula 1 inaccordance with the present invention contains at least oneintramolecular nitrogen atom, the lone electron pair on the nitrogenatom causes intra- or intermolecular interaction. In this mechanism, thepolyamide of the present invention can be greatly improved in solubilityand mechanical strength while retaining its excellent intrinsicproperties.

The acid dianhydride according to the present invention may be preparedby a simple organic synthesis method, such as Michael addition andhydrolysis.

Briefly, the acid dianhydride according to the present invention isprepared by reacting a compound represented by Chemical Formula 2 withpiperazine to give a compound represented by Chemical Formula 3,hydrolyzing the compound of Chemical Formula 3 in the presence of a basecatalyst to obtain a compound represented by Chemical Formula 4, andring closing by dehydration with a dehydrating agent.

This process is summarized as illustrated in the following ReactionScheme 1.

First, a compound of Chemical Formula 2 (dimethyl fumarate) is allowedto undergo the Michael addition reaction with piperazine to form acompound of Chemical Formula 3. In this reaction, the compound ofChemical Formula 2 (dimethyl fumarate) acts as a Michael acceptor whilepiperazine serves as a Michael donor.

Preferably, the Michael addition reaction is conducted at 20˜140° C. for4˜16 hrs in terms of reaction efficiency.

For use in preparing the compound of Chemical Formula 3, the compound ofChemical Formula 2 and piperazine are used at a molar ratio of 1:0.45 to1:0.55. When piperazine is below 0.45 moles or over 0.55 moles per moleof the compound of Chemical Formula 2, the yield may decrease.

There are various ways known to prepare the compound of Chemical Formula2. In a preferred embodiment, fumaric acid is refluxed in methanol inthe presence of an acid catalyst, such as sulfuric acid, followed byneutralization with a neutralizer, such as sodium carbonate, to affordthe compound.

In the present invention, the reaction substrate itself is preferablyused as a solvent, but a separate solvent may be employed. No particularlimitations are imposed on the reaction solvent if it does not interruptthe reaction. For example, 1,4-dioxane, toluene, NMP(N-Methyl-2-pyrrolidone), or DMAc (dimethylacetamide) may be used.

Then, the compound of Chemical Formula 3 may be hydrolyzed in thepresence of a base catalyst to a compound of Chemical Formula 4. Thehydrolysis is conducted at 40˜120° C. for 1˜6 hrs. At less than 40° C.or for less than 1 hr, the reaction does not proceed sufficiently, thusleaving much of the reactants unreacted. At higher than 120° C. or formore than 6 hrs, the solvent or the catalyst may evaporate, which isundesirable in terms of cost and efficiency.

The base catalyst useful for the hydrolysis reaction may be selectedfrom the group consisting of potassium hydroxide, sodium hydroxide,barium hydroxide, calcium hydroxide, magnesium hydroxide, and acombination thereof. Preferred is potassium hydroxide or sodiumhydroxide in view of cost and ease of handling.

Based on 1 mole of the compound of Chemical Formula 3, the base catalystmay be used in an amount of 5 to 10 moles. When the amount of the basecatalyst is less than 5 moles per mole of the compound of ChemicalFormula 3, hydrolysis is difficult to perform. On the other hand, morethan 10 moles of the base catalyst leaves much hydrochloride to bedeposited, which has negative effects on efficiency and productivity.

The compound of Chemical Formula 4 is ring closed by dehydration in thepresence of a dehydrating agent to afford the aliphatic acid dianhydriderepresented by Chemical Formula 1. The ring-closing dehydration isconducted at 40˜100° C. for 4˜28 hrs. At higher than 100° C. or forlonger than 28 hrs, the catalyst or the solvent is prone to evaporating,thus decreasing the yield. At lower than 40° C., the reaction should beconducted for a prolonged time, or if conducted within 4 hrs, thereaction does not proceed sufficiently, decreasing the yield.

The dehydrating agent may be at least one selected from the groupconsisting of acetic anhydride, and a tertiary amine such as pyridine,isoquinoline and triethylamine. In terms of efficiency, acetic anhydrideand/or pyridine is preferred.

Based on 1 mole of the compound of Chemical Formula 4, the dehydratingagent may be used in an amount of 2 moles or more, and preferably in anamount of 2 to 10 moles. When the amount of the dehydrating agent isless than 2 moles per mole of the compound of Chemical Formula 4, thereaction does not proceed sufficiently with the resultant decrease ofthe yield. More than 10 moles of the dehydrating agent isdisadvantageous in view of cost.

After completion of the above-mentioned reactions, the acid dianhydrideis obtained by typical filtration and drying.

In accordance with another aspect thereof, the present inventionaddresses a polyamic acid prepared by reacting an acid dianhydriderepresented by the following Chemical Formula 1 with diamine.

Also, contemplated in accordance with a further aspect of the presentinvention is a polyimide obtained by ring-closing dehydration of thepolyamic acid.

The acid dianhydride of Chemical Formula 1 according to the presentinvention can be polycondensed with diamine to give a polyamic acid thatcan be then subjected to ring-closing dehydration under heat or in thepresence of a catalyst to afford a polyimide.

Without being particularly limited, the diamine may be one of variousdiamines typically used in the synthesis of polyimide. Concrete examplesof the diamine include: aromatic diamines, such as p-phenylene diamine,m-phenylene diamine, 2,5-diaminotoluene, 2,6-diaminotoluene,1,3-bis(4,4′-aminophenoxy)benzene, 4,4′-diamino-1,5-phenoxypentane,4,4′-diaminobiphenyl, 3,3′-dimethyl-4,4′-diaminobiphenyl,3,3′-dimethoxy-4,4′-diaminobiphenyl, 4,4′-diaminodiphenylether,4,4′-diaminodiphenylmethane, 2,2′-diaminodiphenylpropane,bis(3,5-diethyl-4-aminophenyl)methane, diaminodiphenylsulfone,diaminobenzophenone, diaminonaphthalene, 1,4-bis(4-aminophenoxy)benzene,1,4-bis(4-aminophenyl)benzene, 9,10-bis(4-aminophenyl)anthracene,1,3-bis(4-aminophenoxy)benzene, 4,4′-bis(4-aminophenoxy)diphenylsulfone,2,2-bis[4-(4-aminophenoxy)phenyl]propane, and2,2′-trifluoromethyl-4,4′-diaminobiphenyl; cyclic diamines, such as1,4-diaminocyclohexane, 1,4-cyclohexane bis(methylamine), and4,4′-diaminodicyclohexyl methane; and aliphatic diamines, such astetramethylene diamine, and hexamethylene diamine. These diamines may beused solely or in combination.

No particular limitations are imposed on the preparation of the polyamicacid of the present invention. It may be achieved by reacting andpolymerizing the acid dianhydride of Chemical Formula 1 with diamine ina typical process. For convenience, an aliphatic dianhydride representedby Chemical Formula 1 and diamine may be mixed and reacted in an organicsolvent.

Examples of the organic solvent include m-cresol, N-methyl-2-pyrrolidone(NMP), N,N-dimethyl formamide (DMF), N,N-dimethyl acetamide (DMAc),N-methyl caprolactam, dimethyl sulfoxide (DMSO), tetramethyl urea,pyridine, dimethylsulfone, hexamethyl phosphoramide, andγ-butyrolactone. These solvents may be used solely or in combination.Unable as it is to dissolve polyamic acid, an additional solvent may beused in addition to the solvent if it allows for the formation of auniform solution.

The solution polymerization may be conducted at −20˜150° C., andpreferably at −5˜100° C. In addition, the molecular weight of thepolyamic acid may be controlled by adjusting the molar ratio of the aciddianhydride of Chemical Formula 1 to the diamine. A molar ratio nearerto 1 results in a higher molecular weight of the produced polyamic acid.

The polyimide of the present invention is obtained from the polyamicacid prepared above through ring-closing dehydration. Here, a conversionrate of polyimide from polyamic acid (ring-closing dehydration rate) isdefined as a imidization rate. The rate of imidization for the polyimideof the present invention is not limited to 100%, and may be given avalue of 1˜100%, as needed.

Without being particularly limited, the ring-closing dehydration ofpolyamic acid may be achieved typically by heating or chemically with aknown ring-closing dehydration catalyst. The ring-closing dehydration byheating may be conducted at 100˜300° C., and preferably at 120˜250° C.

As for the chemical method, its ring-closing dehydration may beconducted in the presence of a catalyst such as an organic base such aspyridine or triethyl amine, or an acetic anhydride. In this regard, thereaction temperature may be at −20˜200° C. In this reaction, thepolymerization solution of polyamic acid may be used as it is or afterit is diluted. Alternatively, the polyamic acid may be dissolved in asuitable organic solvent after being recovered from the polymerizationsolution as described later. The organic solvent may be the same as isused for the polymerization of polyamic acid.

The polyimide (containing) solution thus obtained may be used as it isor may be added with methanol or ethanol to precipitate the polymer.This polymer may be isolated as a powder or may be re-dissolved in asuitable solvent. Any solvent can be used without limitations if it candissolve the polymer. Selection for the solvent may be made from among,for example, m-cresol, 2-pyrrolidone, NMP, N-ethyl-2-pyrrolidone,N-vinyl-2-pyrrolidone, DMAc, DMF(dimethylformamide), andγ-butyrolactone.

A better understanding of the present invention may be obtained throughthe following examples that are set forth to illustrate, but are not tobe construed as limiting the present invention.

EXAMPLE 1

1-1: Synthesis of Compound of Chemical Formula 2

To a solution of 14.5 g (125 mmol) of fumaric acid in 200 ml of methanolwas added 5 ml of conc. sulfuric acid, followed by refluxing at 100° C.for 1 hr. The reflux was cooled in a cooling bath and neutralized with200 g of 10% sodium carbonate to form a white precipitate. Thisprecipitate was filtered, washed with water, and dried at 50° C. for 12hrs in a vacuum oven to afford the compound of Chemical Formula 2. 16.6g (yield 92%). The preparation of the compound of Chemical Formula 2 wasbased on the method reported by Carr G. et al. (Carr G., Williams D. E.,Di′az-Marrero A. R., Patrick B. O., Bottriell H., Balgi A. D., DonohueE., Roberge M., and Andersen R. J., J. Nat. Prod. 2010, 73, 422).

The compound of Chemical Formula 2 obtained was measured for meltingpoint (Buchi, M-560). Its melting point (Mp) was observed to be 102° C.,which is identical to that of the compound reported by Carr G. et al.

1-2: Synthesis of Compound of Chemical Formula 3

To 100 ml of 1,4-dioxane were added 14.4 g (0.1 mol) of the compound ofChemical Formula 2 obtained in Example 1-1 and 4.3 g (0.05 mol) ofpiperazine, followed by refluxing for 16 hrs. The reflux was cooled toform a precipitate. This precipitate was subjected to many rounds offiltration and condensation, and dried for 12 hrs in a vacuum oven toafford the compound of Chemical Formula 3 as a colorless solid. 15.3 g(yield 82%).

The compound of Chemical Formula 3 obtained was measured for meltingpoint (Buchi, M-560), and analyzed by NMR(¹H and ¹³C) (JEOL, JNM-LA400)and IR (AVATAR, 360 FT-IR).

m. p.: 158° C. (EtOAc)

¹H NMR (400 MHz, CDCl₃): δ 2.38-2.50 (m, 4H, Het-CH₂CH₂), 2.54-2.70 (m,6H, β-CH₂, Het-CH₂CH₂), 2.81 (dd, J=16.0 and 9.2 Hz, 2H, β-CH₂), 3.64(s, 6H, 2OCH₃), 3.68 (dd, 2H, overlapped signals, α-CH), 3.70 (s, 6H,2OCH₃) (FIG. 4); ¹³C NMR (100 MHz, CDCl₃): δ 171.7 and 170.9 (ester C),63.4 (α-CH), 51.8 (OCH₃), 51.5 (OCH₃), 49.9 (Het-CH₂ CH₂), 34.0 (β-CH₂);Anal. Calcd. for C₁₆H₂₆N₂O₈; C: 51.33; H: 7.00; N: 7.48%. Found: C:51.19; H: 7.09; N: 7.53%; IR (KBr, cm⁻¹): 1734 for νC═O

1-3: Synthesis of Compound of Chemical Formula 4

In a 500 ml round-bottom flask, 11.2 g (0.03 mol) of the compound ofChemical Formula 3 obtained in Example 1-2 was mixed with 120 ml (0.24mol) of 2 N potassium hydroxide and 120 ml of methanol. The mixture washeated at 60° C. to the degree of complete dissolution and then for anadditional 3 hrs at the same temperature. After its pH was adjusted to3.8 with conc. HCl, the solution was stirred at room temperature for 30min to form a precipitate. This precipitate was filtered, and washedwith water. Then, the precipitate was dried for 24 hrs in a vacuum oven,and re-crystallized in 2,000 ml of a mixture of 1:1 water and methanolto afford the compound of Chemical Formula 4. 8.7 g (yield 92%). Sincethe compound of Chemical Formula 4 is not dissolved in general organicsolvents nor in water, an NMR sample was prepared by mixing the compoundof Chemical Formula 4 with heavy water (D₂O) in which solid potassiumhydroxide was dissolved. The compound of Chemical Formula 3 obtained wasmeasured for melting point (Buchi, M-560), and analyzed by NMR (¹H and¹³C) (JEOL, JNM-LA400) and IR (AVATAR, 360 FT-IR).

m.p.: 218-219° C. (H₂O+MeOH)

*¹H NMR (400 MHz, D₂O/KOH): δ 2.46-2.54 (bd, 4H, Het-CH₂CH₂), 2.56-3.40(bm, 6H, β-CH₂, Het-CH₂CH₂), 3.44-3.54 (bm, 2H, β-CH₂), 4.80 (dd, 2H,overlapped signals, α-CH); ¹³C NMR (100 MHz, D₂O/KOH): δ 178.8 (COOH),67.7 (α-CH), 49.1 (Het-CH₂ CH₂), 37.4 (β-CH₂); Anal. Calcd. forC₁₂H₁₈N₂O₈; C: 45.28; H: 5.70; N: 8.80%. Found: C: 45.16; H: 5.79; N:8.83%; IR (KBr, cm⁻¹): 1723 for νC═O.

1-4: Synthesis of Compound of Chemical Formula 1

In a 50 ml flask equipped with a magnetic stirrer, 5.4 g (17 mmol) ofthe compound of Chemical Formula 4 obtained in Example 1-3, 3.18 g (35.7mmol) of pyridine, and 3.6 g (35.7 mmol) of acetic anhydride wereplaced, and allowed to react at 60° C. for 24 hrs. After completion ofthe reaction, the reaction mixture was cooled and filtered. The filtratewas washed with 200 ml of acetic anhydride and 200 ml of purifieddiethylether, and dried at 40° C. in a vacuum oven. Recrystallization in100 ml of acetic anhydride produced the compound of Chemical Formula 1.2.9 g (yield 60%).

The compound of Chemical Formula 1 obtained was measured for meltingpoint (Buchi, M-560), and analyzed by NMR (¹H and ¹³C) (JEOL, JNM-LA400)and IR (AVATAR, 360 FT-IR).

m. p.: 156° C. (decompose)

¹H NMR (400 MHz, d6-DMSO): δ 2.39 (bd, J=6.8 Hz, 4H, Het-CH₂CH₂), 2.76(bd, J=6.8 Hz, 4H, Het-CH₂CH₂), 3.04 (d, J=8.4 Hz, 4H, β-CH₂), 4.21 (t,J=16.4 Hz, 2H, α-CH); ¹³C NMR (100 MHz, d6-DMSO): δ 171.6 (COOCO), 170.6(COOCO), 63.6 (α-CH), 48.9 (Het-CH₂CH₂), 31.9 (β-CH₂); Anal. Calcd. forC₁₂H₁₄N₂O₆; C: 51.06, H: 5.00, N: 9.93%. Found: C: 49.93, H: 5.10, N:9.96%; IR (KBr, cm⁻¹): 1860, 1781 (νC═O), 1210, 1127 (C—O—C)

Although the preferred embodiment(s) of the present invention have(has)been disclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

The invention claimed is:
 1. An acid dianhydride, represented by thefollowing Chemical Formula 1:


2. A method for preparing an acid dianhydride, represented by thefollowing Chemical Formula 1, comprising: (a) reacting a compoundrepresented by the following Chemical Formula 1 with piperazine to givea compound represented by the following Chemical Formula 3; (b)hydrolyzing the compound of Chemical Formula 3 in the presence of a basecatalyst to obtain a compound represented by Chemical Formula 4; and (c)subjecting the compound of Chemical Formula 4 to ring-closingdehydration in presence of a dehydrating agent:


3. The method of claim 2, wherein the reaction of step (a) is a Michaeladdition reaction.
 4. The method of claim 2, wherein the compound ofChemical Formula 2 is reacted at a molar ratio 1:0.45 to 1:0.55 withpiperazine in step (a).
 5. The method of claim 2, wherein the basecatalyst of step (b) is selected from the group consisting of potassiumhydroxide, sodium hydroxide, barium hydroxide, calcium hydroxide,aluminum hydroxide, magnesium hydroxide, and a combination thereof. 6.The method of claim 2, wherein the hydrolysis of step (b) is conductedat 40˜120° C. for 1˜6 hrs.
 7. The method of claim 2, wherein thedehydrating agent of step (c) is selected from the group consisting ofacetic anhydride, pyridine, isoquinoline, a tertiary amine, and acombination thereof.
 8. The method of claim 2, wherein the ring-closingdehydration of step (c) is conducted at 40˜100° C. for 4˜28 hrs.
 9. Themethod of claim 2, wherein the dehydrating agent of step (c) is used inan amount of 2˜10 moles per mole of the compound of Chemical Formula 4.